In a contemporary disc drive, a transducer records information onto a magnetic disc in concentric tracks. Each piece of data that is recorded on the magnetic disc is assigned a location. When the information is needed, the transducer must return to the exact location and track where the piece of data has been stored.
As track densities have increased, disc drives have become more sensitive to vibrations which deflect the transducer from the track it follows or which cause the magnetic disc to vibrate beneath the transducer. In effect, vibrations within the disc drive cause the disc to move or slip underneath the transducer. Motion of the magnetic disc relative to the transducer can cause the transducer to slip further along the track producing read/write errors. Furthermore, a contemporary disc drive needs to meet exacting standards with respect to the speed with which data can be accessed and recorded. Movement of the magnetic disc relative to the transducer slows down both information retrieval times and data recording times for the system. There exists a need to detect and compensate for these vibrations before they cause slipping of the magnetic disc.
Rotational accelerations as low as 21 radians/second.sup.2 can cause track slipping. One source of rotational vibration involves disc drives stacked in close proximity to each other. An actuator arm controls the movement of the transducer relative to the magnetic disc for each disc drive. During a seeking mode, the actuator arm of a disc drive will move the transducer rapidly over the surface of the magnetic disc. The rapid movement of the actuator arms in such close proximity to other disc drives can cause rotational vibrations which affect the track following performance of nearby disc drives. When dozens of disc drives are stacked together, the effect can be significant.
Several solutions to this problem have been suggested. Dedicated servo surface systems attempt to maintain constant information regarding the transducer's position relative to the magnetic disc by dedicating a portion of the magnetic disc space to storing this information. This information is then used by a servo control system to compensate for track skipping during use. This solution suffers from the obvious disadvantage of consuming disk space which would otherwise be available for other data.
Embedded servo surface systems embed periodic reference points on the surface of the magnetic disk to provide the system with position information. This system requires less disc surface space than the dedicated servo surface systems, but they do not provide constant position information. Embedded reference points only provide position information periodically as the transducer passes over a reference point. Therefore, embedded servo surface systems do not provide instantaneous and constant position information.